It is not documented because it is not possible with Qt (neither user nor owner password). You could post-process the file to encrypt it (user password) with reference to the relevant "PDF specification":http://www.adobe.com/devnet/pdf/pdf_reference.html or use a third-party library to generate the PDF in the first place.

Yes, you would need to parse the PDF file, encrypt the string objects and content streams using the weak RC4 scheme or AES (or another option) and rewrite the file with the necessary extra dictionary entries describing the encryption/document permissions.